Control systems for nuclear reactors require the monitoring of temperature and pressure in the reactor core to determine the condition of the coolant flowing in the nuclear reactor. Depending upon the temperature and the pressure of the coolant, the coolant may exist either in a purely liquid form, purely vapor or steam form, or a combination of liquid and vapor of the liquid. Curves are known relating temperature and pressure of the coolant wherein the curve is called the saturation curve. An example of such a known curve may be seen in FIG. 1. The area above and to the left of the saturation curve is called the sub-cooled region. In this region of the curve, the cooling fluid exists purely in its liquid form. The area below and to the right of the saturation curve is called the superheat region and in this region the cooling fluid exists as a vapor or steam. Anywhere on the saturation curve, the cooling fluid may exist as a combination of liquid and its vapor.
Prior art indicating systems utilized the measurement of coolant temperature and pressure individually. It may be seen from the above description that knowledge of the coolant fluid state requires combining of pressure and temperature measurement to obtain an indication of the margin of subcooling. This can be done manually by the operator. However, it is desirable to have this important measure of system condition directly available from an instrument which accurately monitors the individual variable and represents the margin directly. Such an instrument would provide an indication to the reactor operator which would tell him the margin to the saturation curve taking into consideration both the actual measured temperature and the pressure of the cooling fluid.